Transparent overlapping LED die layers

ABSTRACT

A first layer of inorganic first vertical LED dies (VLEDs) of a first color is printed on a conductor surface. A first transparent conductor layer is deposited over the first VLEDs to electrically contact top electrodes of the first VLEDs. An electrically insulated second layer of second VLEDs of a second color is printed over the first transparent conductor layer, and an electrically insulated third layer of third VLEDs of a third color is printed over the first transparent conductor layer. For a color display, the VLEDs are printed in an addressable pixel array. Since the VLEDs are printed as an ink, the overlying VLEDs in a pixel are not vertically aligned, so there is little blockage of light. If the structure is used for general illumination, the VLEDs do not need to be printed in pixel areas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.14/150,199, filed Jan. 8, 2014, by Bradley Steven Oraw, which claimspriority to U.S. provisional application Ser. No. 61/759,137 filed Jan.31, 2013 by Bradley Steven Oraw, both assigned to the present assigneeand incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to forming a plurality of overlapping,transparent light emitting diode (LED) layers and, in particular, to aprinting method for forming such LED layers using vertical printing ofmicroscopic LED dies.

BACKGROUND

FIG. 1 generally illustrates a prior art technique for forming a widearea light sheet or panel using LEDs. A starting substrate 10 may beMylar or other type of polymer sheet, or even a metal sheet. A conductorlayer 12 is then deposited over the substrate 10, such as by printing.The substrate 10 and/or conductor layer 12 is preferably reflective. Areflective film may also be provided on the front or back surface of thesubstrate 10.

An LED ink is provided, which comprises microscopic vertical LEDs 14(e.g., 30 microns in diameter) uniformly infused in a solvent. The LEDsare initially formed as metallized semiconductor layers on a carrierwafer. Trenches are photolithographically etched through thesemiconductors layers and the metal layers to define the boundaries ofeach LED. The back surface of the carrier wafer is then thinned untilthe individual LEDs are separated. The LEDs are then dispersed in thesolvent to create the ink.

The LED ink is screen printed over the conductor layer 12. Theorientation of the LEDs can be controlled by providing a relatively talltop electrode 16 (e.g., the anode electrode), so that the top electrode16 orients upward by taking the fluid path of least resistance throughthe solvent after printing. The LED ink is heated to evaporate thesolvent, and the bottom cathode electrode 18 and conductor layer 12 areannealed to create an ohmic cathode connection.

A dielectric 19 is deposited and etched to expose the top electrode 16.

A transparent conductor layer 20 is then printed to contact the topelectrodes 16.

Metal bus bars 22 and 24 are then printed and cured to electricallycontact the conductor layers 12 and 20 along their edges. A suitablevoltage differential applied to the bus bars 22/24 turns on the LEDs 14.Although the microscopic LEDs 14 are randomly distributed, they arefairly uniformly distributed over the area of the flat sheet due to thelarge number of LEDs printed. There may be millions of LEDs 14 printedon a one square meter substrate 10. The fabrication process may beperformed under atmospheric conditions.

The LEDs 14 in the monolayer, within a defined area, are connected inparallel by the conductor layers 12/20 since the LEDs 14 have the sameorientation. If many LEDs 14 are connected in parallel, the drivingvoltage must approximately equal the voltage drop of a single LED 14 andthe current is relatively high. The high current flowing laterallythrough at least the thin transparent conductor layer 20 creates asignificant IR drop, since typical transparent conductors may have aconductivity of 1 ohm/square. This results in power loss and heat,lowering the efficiency of the lamp. Making the transparent conductorlayer 20 thicker adds cost and increases the light absorption by thelayer 20.

Further detail of forming such a light source by printing microscopicvertical LEDs, and controlling their orientation on a substrate, can befound in US application publication US 2012/0164796, entitled, Method ofManufacturing a Printable Composition of Liquid or Gel Suspension ofDiodes, assigned to the present assignee and incorporated herein byreference.

It is common to connect discrete LEDs in series by using printed circuitboards and other techniques. By connecting LEDs in series, the drivingvoltage increases and the driving current is lowered. However, suchelectrical interconnections are impractical for printed LEDs, since theLEDs are randomly positioned and microscopic. Further, using lateralconductors to connect a layer of LEDs in series uses significantsubstrate area, creating noticeable dark areas between the LEDs andlowering the brightness-to-area ratio of the lamp.

What is needed is a practical and cost-effective technique forconnecting printed LEDs in series while still obtaining a high densityof LEDs for a good brightness-to-area ratio.

What is also needed are techniques that make use of the transparency ofthe LED die layers to create other types of LED devices, such as colordisplays.

SUMMARY

In one embodiment, microscopic LEDs are infused in a solvent to form anLED ink for printing, such as screen printing. The LEDs are verticalLEDs, with one electrode (e.g., an anode) on top and the other electrode(e.g., a cathode) on the bottom.

A substrate is provided with a first conductor layer. The LEDs areprinted, as a monolayer, on the conductor layer, with their anodeelectrodes orientated up, and the bottom cathode electrodes are annealedto make ohmic contact to the first conductor layer.

A dielectric is deposited over the LEDs, followed by a transparentsecond conductor layer, which makes ohmic contact with all of the topanode electrodes. Therefore, this first layer of LEDs is connected inparallel by the first and second conductor layers.

Over the second conductor layer is printed a second monolayer of LEDs,which may be identical to the first layer of LEDs. A dielectric isdeposited over the second layer of LEDs, and a transparent thirdconductor layer is deposited, which makes ohmic contact to the anodeelectrodes of the second layer of LEDs. Therefore, the second layer ofLEDs is connected in parallel by the second conductor layer and thethird conductor layer.

The first layer of LEDs is connected in series with the second layer ofLEDs, since the anode electrodes of the first layer of LEDs areconnected to the cathode electrodes of the second layer of LEDs via thesecond conductor layer. Since the LEDs are randomly arranged in eachlayer (i.e., not vertically aligned), the light from the LEDs in thefirst layer will typically have a direct path through the overlyingdielectric layer and transparent conductor layers.

The transparent second conductor layer may be made very thin since, mostof the current through the second conductor layer flows vertically andonly slightly laterally until it is conducted by an LED in the secondLED layer. Therefore, there is little light attenuation by the secondconductor layer and very little power loss.

Metal bus bars are printed along opposite edges of all the conductorlayers for good current distribution across the conductor layers. Thinmetal runners may be printed between the bus bars to improve the currentdistribution for large area light sheets.

A driving voltage is coupled between the first conductor layer and thethird conductor layer to turn on all the LEDs with the properorientation. Since the LEDs are in series, the current conducted by theconductor layers is one half of the current which would have beenconducted had all the LEDs been connected in parallel.

Additional LED layers may be printed to connect additional LEDs inseries and further lower the current.

By lowering the current through the conductors, the conductors have alower IR drop and/or may be made thinner.

Additionally, since the overall density of the series-connected LEDs isgreater than the density of lateral-connected LEDs (since theseries-connected LED layers overlap), the brightness-to-area ratio isgreatly increased.

In another embodiment, a dielectric layer may be inserted over thetransparent second conductor layer, and another transparent conductorlayer may be printed over the dielectric layer for contacting the bottomcathode electrodes of the second layer of LEDs. This allows completelyindependent control over the driving of the first layer of LEDs and thesecond layer of LEDs. This also provides the option of externallyconnecting the LEDs in series.

By independently driving the layers of LEDs, the LEDs may be differenttypes and colors, and the overall combined color may be dynamicallycontrolled. Therefore, full color pixels may be formed by overlappingsmall areas of red, green, and blue LEDs, or phosphor-converted LEDs,where the RGB LED dies in each pixel are addressable to control pixelcolor. In this way, a very large, roll-up flexible color display can beformed by printing.

A phosphor or quantum dot layer may be added to wavelength-convert someof the LED light to generate any color.

The light sheet may be fabricated using a conveyor system at atmosphericpressures.

Other embodiments are described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-section of a prior art light sheet with anarray of vertical LEDs sandwiched between two conductor layers toconnect the LEDs in parallel.

FIG. 2 is a simplified cross-section of a light sheet in accordance withone embodiment of the invention during fabrication.

FIG. 3 illustrates the light sheet of FIG. 2 after additionalfabrication steps to form two layers of LEDs in series.

FIG. 4 is a top down view of only the top layer of LEDs in FIG. 3, whereFIG. 3 is taken along line 3-3 in FIG. 4.

FIG. 5 illustrates a current path through two LEDs in series and alsoillustrates light rays emitted from the top surfaces of two LEDs.

FIG. 6 illustrates another embodiment of the invention using twovoltage/current sources to drive the respective LED layers forindependent control of the two layers.

FIG. 7 illustrates another embodiment of the invention where there is adielectric layer between the two middle conductor layers to enable avariety of electrical connections between the two LED layers.

FIG. 8 illustrates how the embodiments may be inexpensively and quicklyfabricated using a conveyor system at atmospheric pressures.

FIG. 9 is a cross-section of a small portion of a color display, orcolor-adjustable lamp, where transparent layers of red, green, and blueLED dies, or phosphor-converted LED dies, overlap, and where, for acolor display, the LED dies are printed in pixel locations such thatvarious pixel colors are achievable.

FIG. 10 is a front view of a portion of a color display showing RGBpixels.

Elements that are similar or identical in the various figures arelabeled with the same numeral.

DETAILED DESCRIPTION

The GaN-based micro-LEDs used in embodiments of the present inventionare less than a third the diameter of human hair, rendering themessentially invisible to the naked eye when the LEDs are sparsely spreadacross a substrate to be illuminated. The number of micro-LED devicesper unit area may be freely adjusted when applying the micro-LEDs to thesubstrate. A well dispersed random distribution across the surface canproduce nearly any desirable surface brightness. Lamps well in excess of10,000 cd/m² have been demonstrated by the assignee. The LEDs may beinfused in a solvent and printed as an ink using screen printing orother forms of printing, and the orientations of the LEDs on thesubstrate are controllable by, for example, the shape of the topelectrode. A tall electrode causes the tall electrode to be the topelectrode about 90% of the time. Further detail of forming a lightsource by printing a monolayer of microscopic vertical LEDs, controllingtheir orientation on a substrate, and connecting them in parallel can befound in US application publication US 2012/0164796, entitled, Method ofManufacturing a Printable Composition of Liquid or Gel Suspension ofDiodes, assigned to the present assignee and incorporated herein byreference.

In contrast to the prior art FIG. 1, the present invention forms anynumber of printed LEDs in series by printing overlapping layers of LEDsto reduce the current through the conductor layers and increase thedensity of LEDs to increase the brightness-to-area ratio.

FIG. 2 illustrates one embodiment of an inventive light sheet (or panel)during fabrication.

In one embodiment, an LED wafer, containing many thousands of verticalLEDs, is fabricated so that the metal anode electrode 16 for each LED 14has a small footprint to allow light to escape the anode side, and themetal cathode electrode 18 for each LED 14 forms a bottom reflector foreach LED 14. The anode and cathode surfaces may be opposite to thoseshown. The top surface of the LEDs 14 may be roughened by etching toincrease light extraction (i.e., decrease internal reflections).

The LEDs 14 are completely formed on the wafer, including the anode andcathode metallizations, by using one or more carrier wafers during theprocessing and removing the growth substrate to gain access to both LEDsurfaces for metallization. After the LEDs are formed on the wafer,trenches are photolithographically defined and etched in the frontsurface of the wafer around each LED, to a depth equal to the bottomelectrode, so that each LED has a diameter of about 30 microns and athickness of about 6 microns. A preferred shape of each LED ishexagonal. The back surface of the wafer is then thinned until the LEDsare singulated. The LEDs 14 of FIG. 2 result. The microscopic LEDs 14are then uniformly infused in a solvent to form an LED ink for printing,such as screen printing.

Many other ways can be used to form the LEDs 14, and the LEDs 14 do notneed to be microscopic or printed for the present invention to apply.

If it is desired for the anode electrodes 16 to be oriented in adirection opposite to the substrate 10 after printing, the electrodes 16are made tall so that the LEDs are rotated in the solvent, by fluidpressure, as they settle on the substrate surface. The LEDs rotate to anorientation of least resistance. Over 90% like orientation has beenachieved, although satisfactory performance may be achieved with over75% of the LEDs being in the same orientation.

The starting substrate 10 is preferably as thin as practical for ease ofhandling, low weight, and low cost, and provides suitable mechanicalstrength to the light sheet. The substrate 10 may be a suitable polymer,such as Mylar or PMMA, and may be dispensed from a roll. The substrate10 can be any size, such as one square meter across, since no vacuumprocessing is needed for fabrication, and the process may be performedusing a conveyor system. In one embodiment, the bottom surface of thesubstrate 10 has a thin reflective metal film deposited on it.

On top of the substrate 10 is deposited a conductor layer 12, such as ametal layer formed of copper, aluminum, or an alloy. The conductor layer12 is preferably reflective to visible light.

The LEDs 14 are then printed on the conductor layer 12 such as by screenprinting with a suitable mesh to allow the LEDs to pass through andcontrol the thickness of the layer. The LEDs 14 will be printed as amonolayer and be fairly uniformly distributed over the substrate 10. Anyother suitable deposition process may be used. In the example of FIG. 2,the LEDs 14 are oriented so that the anode electrodes 16 are the topelectrodes.

The LED ink solvent is then evaporated by heat, such as using lamps. TheLEDs 14 are then annealed in an optical furnace, also using lamps, torapidly heat the electrodes to form an ohmic connection between thebottom cathode electrodes 18 and the conductor layer 12. Millions ofLEDs 14 may be deposited in the single layer.

A transparent dielectric layer 19 is then printed over the entiresurface to encapsulate the LEDs 14 and secure them in position. The topsurface of the dielectric layer 19 is then blanket etched, such as by awet etching or mechanical etching, to expose the top anode electrodes16.

A transparent conductor layer 20 is then printed over the dielectriclayer 19 to electrically contact the electrodes 16. The conductor layer20 may be ITO or may include silver nanowires. The electrodes 16 may beannealed in an optical furnace to create good ohmic contact to theconductor layer 20.

As shown in FIG. 3, another transparent conductor layer 32 is printedover the conductor layer 20.

The LED ink is then again printed over the conductor layer 32 to form asecond layer of LEDs 14, which may be identical to the LEDs 14 in thefirst layer or different. In one embodiment, all the LEDs 14 are thesame and emit blue light. A phosphor layer, to be later deposited, isused to cause the light sheet to emit white light or any other color.

In another embodiment, the conductor layers 20 and 32 are formed as asingle layer, and the electrodes in the first and second layers of LEDs14 are annealed in a single step to make ohmic contact to both sides ofthe conductor layer.

The following steps may be identical to those described with respect toFIG. 2. The LED ink solvent is then evaporated by heat, such as usinglamps. The LEDs 14 in the second layer are then annealed in an opticalfurnace to rapidly heat the electrodes to form an ohmic connectionbetween the bottom cathode electrodes 18 and the conductor layer 32.

A transparent dielectric layer 34 is then printed over the entiresurface to encapsulate the LEDs 14 and secure them in position. The topsurface of the dielectric layer 34 is then blanket etched, such as by awet etching or mechanical etching, to expose the top anode electrodes16.

A transparent conductor layer 36 is then printed over the dielectriclayer 34 to electrically contact the electrodes 16. The conductor layer36 may be ITO or may include silver nanowires. The electrodes 16 may beannealed in an optical furnace to create good ohmic contact to theconductor layer 36.

The LEDs 14 in each layer are thus connected in parallel, and the twolayers of LEDs 14 are connected in series. Additional overlapping layersof LEDs 14 may be printed to add more LEDs in series.

The various layers are printed so that edge areas of the conductorlayers 12, 32, and 36 are exposed.

Metal bus bars 40-45 are then screen printed along opposite edges of theconductor layers 12, 32, and 36 for connection to one or morevoltage/current sources. The metal is then annealed by an opticalfurnace. The bus bars will ultimately be connected to a voltagedifferential for turning on the LEDs 14. The points of connectionbetween the bus bars and the driving voltage leads should be at least ontwo ends of each bus bar to more uniformly distribute current along thebus bars. The bus bars on opposite edges of a conductor layer areshorted together, either by the printed metal or an external connection.

The bus bars 42 and 43, connected to the middle conductor layer 32, areoptional if the two layers of LEDs 14 are to be driven by a singlevoltage/current source.

FIG. 4 is a top down view of the structure of FIG. 3, where FIG. 3 istaken along line 3-3 in FIG. 4. Only the second layer of LEDs 14 isillustrated for simplicity. The LEDs 14 in the first layer would also bevisible in a top down view, since the various layers are transparent andthe LEDs 14 in the layers would be offset due to their random positionsin the layers.

If the light sheet is wide, there will be a significant IR drop acrossat least the transparent conductor layer 36. Thin metal runners 46 maybe printed along the surface of the conductor layer 36 between the twobus bars 44 and 45 to cause the conductor layer 36 to have a moreuniform voltage, resulting in more uniform current spreading.

The resulting structure may be less than 1 mm thick.

FIG. 5 illustrates the structure of FIG. 3 when a driving voltage 48 isapplied across the conductor layers 12 and 36 to turn all the LEDs 14on. The same driving voltage 48 is applied to the opposite edges of theconductor layers 12 and 36 via the bus bars 40, 41, 44, and 45. Current50 spreads along the bottom conductor layer 12 (which may be metal) andenters the cathode electrodes 18 of each of the LEDs 14. The currentflows vertically through each of the LEDs 14 in the first layer, thenflows both vertically and slightly laterally through the conductorlayers 20 and 32 until conducted by an LED 14 in the second layer ofLEDs 14. The conductor layers 20 and 32 may be very thin since thecurrent through them does not flow laterally a significant distance.Further, the conductor layers 20 and 32 may be made more transparent byreducing their conductivity without significant power loss.

The current 50 flows vertically generally through the nearest overlyingLEDs 14 and completes the circuit through the top transparent conductorlayer 36.

Since the various layers are very thin and transparent, and theconductor layer 12 or the substrate 10 is reflective, there is littlelight absorption. There is also less IR loss since the current suppliedto the conductor layers 12 and 36 is one-half that supplied to theconductor layers 12 and 20 in FIG. 1.

Since the two layers of LEDs 14 in FIG. 5 have twice the density as thesingle layer of LEDs 14 in FIG. 1, the brightness-to-area ratio isdoubled. FIG. 5 illustrates light rays 52 and 54 being emitted from twoLEDs 14. Since the LEDs 14 are extremely small and randomly positioned(not vertically aligned), the LEDs 14 in the second layer will typicallynot directly overlie the LEDs 14 in the first layer. Hence, a majorityof light exiting the LEDs in the first layer is not blocked by the LEDsin the second layer. Further, since the bottom electrodes 18 of the LEDsare reflective, any impinging light is reflected and ultimately exitsthe light sheet. The relative size of the LEDs 14 in FIG. 5 is greatlyexaggerated, and the spacings of the LEDs 14 are greatly compressed forease of illustration.

FIG. 6 illustrates how the driving voltages for the two layers of LEDs14 can be independent by coupling one voltage V1 between the conductorlayers 12 and 32, and coupling another voltage V2 between the conductorlayers 32 and 36. This is useful when the LEDs 14 in the differentlayers are different types of LEDs 14 or it is desired to make one layerof LEDs 14 brighter than the other layer. The current 55 is shownflowing through the series LEDs 14 and to the voltage/current supplies.

FIG. 6 also shows the use of a printed phosphor layer 56 over the topsurface of the conductor layer 36. The phosphor layer 56 passes some ofthe blue LED light and converts a portion of the LED light to anotherwavelength, such as yellow (shown as light ray 58). The combination ofthe light may be white or other color. Other wavelength-convertinglayers may be used, such as a quantum dot layer.

FIG. 7 illustrates the use of an additional transparent dielectric layer58 between the conductor layers 32 and 20. Metal bus bars 60 and 61 areformed on the edges of the conductor layer 20. The dielectric layer 58electrically insulates the two layers of LEDs 14 to create a 4-terminallight sheet. The two layers of LEDs 14 are show driven by the voltagesV1 and V2. The layers of LEDs 14 may be connected in series by anexternal connection, shorting the conductor layer 20 to the conductorlayer 32. The configuration of FIG. 7 may be useful if the LEDs 14 ineach layer are different types of LEDs, requiring different drivingvoltages and currents to produce the desired brightness.

In all the embodiments, a single light sheet may be formed by multipleareas of LEDs tiled on a single substrate, where each separate area ofLEDs comprises LEDs electrically connected in parallel and series by thevarious conductor layers. As an example, one strip of LEDs may beelectrically isolated from an adjacent strip as a result of the patternused during the screen printing of the LEDs and conductor layers. Inthis way, the separate strips may be connected together in series and/orparallel, or isolated, by metal patterns on the light sheet to achievethe desired electrical characteristics of the light sheet. Dividing theLEDs into areas also reduces the required current for each conductorlayer and improves reliability in the event of a short or open circuit.Each strip may be a centimeter wide or less and contain thousands ofLEDs. By enabling driving the strips with different voltages, differenttypes of LEDs (having different forward voltages) may be used inadjacent strips to combine the different colors from the strips. In oneembodiment, red, green, and blue LEDs are in adjacent narrow strips tocreate white light without a phosphor.

A single light sheet may be more than a meter across and any length.Each of the figures may represent a single strip or area in a largerlight sheet or may represent the entire light sheet. The various metalbus bars may be interconnected in any manner.

The substrate 10 may be provided with a release layer to allow theremaining layers to be removed from the substrate 10, creating a moreefficient light sheet having a thickness of only 20-80 microns. Such alight sheet is extremely flexible and may be adhered to another type ofsubstrate, including a fabric for clothing.

The light sheets can be used for general illumination, displays,backlights, indicator lights, etc.

Since all the layers may be printed and heated using lamps, the lightsheet may be manufactured using a conveyor system at atmosphericpressures, as shown in FIG. 8. FIG. 8 illustrates the substrate 10 beingdispensed from a roll 64. The substrate 10 is moved under variousprocess stations to print layers, heat/cure/anneal the layers, and etchthe layers, as described above. Shown in FIG. 8 are a transparentconductor printing station 70, an LED ink printing station 72, a metalprinting station 74, an optical cure/anneal station 76, and a wet or dryetching station 78.

FIG. 9 illustrates a variation in the structure of FIG. 7, where eachtransparent layer of the printed microscopic inorganic vertical LED dies(VLEDs) may be controlled independently, and there are three layers ofred, green, and blue LED dies. In another embodiment, the LED dies mayall be GaN-based and emit blue light from their active layer, but redand green phosphors are used to create the red and green colors.

FIG. 9 is a cross-section of a small portion of a color display showingonly two pixels in a 2-D array of pixels.

Over a flexible conductor layer 80 on a substrate 10, such as thesubstrate 10 in FIG. 7, microscopic red LED dies 82 (or blue LED dieswith a red phosphor) are printed in small pixel areas 84, such as shownby the pixel areas 84 of the color display 85 of FIG. 10. As previouslydescribed, the bottom electrodes of the red LED dies 82 contact theconductor layer 80. A transparent dielectric layer 86 fills the areabetween the red LED dies 82. A transparent conductor layer 88 is thendeposited over the red LED dies 82 to connect each of the red LED dies82 in a pixel area 84 in parallel. The size of the pixel area 84, andthe number of LED dies in each pixel area 84, will depend on the totalsize of the color display and the pixel resolution. For example, eachpixel area 84 may, on average, contain 2-10 LED dies. The exact numberof LED dies per pixel area 84 will be essentially random as a result ofthe printing process but, since the red LED dies 82 are uniformlydistributed in the LED ink, there should be only a small range of LEDdies within each pixel area 84.

A transparent dielectric layer 89 is then deposited to insulate the topof the conductor layer 88.

A similar process is then performed to deposit a transparent conductorlayer 90, green LED dies 92, transparent dielectric layer 93 between thegreen LED dies 92, and a top conductor layer 94 for the green LED dielayer. This is followed by a similar process to form the blue LED dielayer comprising a transparent dielectric layer 96, a transparentconductor layer 98, blue LED dies 100, a transparent dielectric layer101 between the blue LED dies, and a top transparent conductor layer102.

Since the LED dies have an opaque reflective bottom electrode, there islittle downward light emitted from the LED dies. The dielectric layersmay be formed of a resilient material so there will be no delaminationbetween layers if the resulting display is rolled up.

The conductor layers 80, 90, and 98 may form Y conductor strips (goinginto and out of the drawing sheet) to “vertically” address any pixelcolor in any pixel, and the conductor layers 88, 94, and 102 may formorthogonal X conductor strips to “horizontally” address any pixel colorin any pixel. The pixel color at the intersection of energized X and Yconductor strips is addressable to independently control the overallcolor emitted by each pixel 84. The addressing of a particular colorwithin a pixel can be considered a Z address.

The various Y conductor strips for each pixel are electrically isolatedin the “horizontal” direction from the other Y conductor strips, and thevarious X conductor strips for each pixel are electrically isolated inthe “vertical” direction from the other X conductor strips so that theRGB LED dies in each pixel can be independently controlled. This may bedone by screen printing the conductor layers, where the screen has thedesired pixel pattern, or the conductor layers may be cut (ablated)using a laser.

Since the LED dies are printed in a non-deterministic pattern in eachpixel area 84, the LED dies will not directly overlap each other in thevertical direction, so that emitted light is not blocked. Any amount ofexpected blockage by overlapping LED dies can be compensated for whenenergizing the LED dies of the various colors to achieve the desiredcombined pixel color.

Relatively small tiles of pixel arrays may be connected together to forma color display of any size.

FIG. 9 shows a red light ray 106, a green light ray 107, and a bluelight ray 108 being emitted by the three layers of LED dies in theleftmost pixel area 84. The LED dies in each layer may be energizedsimultaneously or energized using time division multiplexing (TDM).Pulse width modulation (PWM) at a frequency above 30 Hz, to avoidperceptible flicker, may be used to create the desired perceivedbrightness of each pixel color, which enables the LED dies to be drivenat the optimal current. Accordingly, both TWM and PWM may be used toachieve the desired pixel color with no flicker.

FIG. 7 illustrates further details of only two of the RGB LED layers,where the conductor layers of each color in a pixel terminate in metaltraces (the bus bars 40, 42, 44, and 60) that act as the X and Yaddressing lines. The metal bus bars may be opaque, such as platedcopper, since they run between the pixels. An opaque layer between thepixels may be printed to reduce cross-talk.

FIG. 9 also illustrates a suitable addressing circuit 110 that applies acurrent through the LED dies in each addressed pixel to create a desiredimage. There is independent drive control of each color in each pixel.Many types of addressing schemes may be used, such as a rapid scanningof the pixels.

A controller 112 may be used to control the addressing circuit 110 usingbuffered image codes that convey the brightness of each pixel color ineach frame.

In another embodiment, the controllable RGB LEDs may serve as acontrollable backlight for a liquid crystal display (LCD), where abacklight pixel is energized at the same time that an associated LCDpixel (acting like a light shutter) is activated to pass selectedcomponents of the RGB light to output a desired color for that pixel.The LCD pixels that are “closed” will have a non-energized backlightpixel behind it. If the LCD uses a scanning sequence to activate itspixels, the backlight uses the same scanning sequence to activate itspixels. In this way, a large amount of power is conserved by notenergizing the entire backlight all the time. Additionally, by notenergizing all the backlight pixels at the same time, darker “blacks”can be displayed.

In another embodiment, pixels are not formed and the RGB LED layers areindependently controlled to produce a desired overall color temperaturefor general illumination or backlighting.

The individual LED dies are less than 200 microns in diameter andpreferably about 30 microns or less in diameter. In a large rectangulardisplay of about five meters wide, there may be 1000 pixels in thehorizontal direction. Each pixel is therefore about 5 mm per side, foran area of 25 mm² per pixel. If each LED die in a pixel in a singlelayer has an area of about 0.1 mm², and there are about five LED dies ineach pixel in a single layer, only about 2% of the area in each pixel ina single layer will consist of LED dies. Therefore, there will be verylittle blockage of light by overlapping LED dies. Since each pixel mayconsist of 15 or more LED dies, high pixel brightness can be obtainedfor outside daytime viewing.

The pixels in the array of pixels are considered substantially identicaleven though there may be a small range of variation in the numbers ofeach type of LED die in each pixel as an inherent result of the printingprocess. If desired, the resulting display can be tested to determinethe differences between the pixels, and compensation factors can beapplied to each pixel color to compensate for the different numbers ofVLEDs in each pixel color. These compensation factors may be applied tothe currents for the pixels or to the PWM of the pixel color. Since theoverlapping LED light sheets can be bent, any shape display or lamp maybe formed. The three LED layers may be less than 1 mm thick.

In one embodiment, the LED dies are printed upside down from theorientation shown in FIG. 7 or FIG. 9, so the light is emitted throughthe transparent substrate 10, such as a clear PET film. In that way, theLED layers are protected by the substrate 10.

The order of colors in the various layers may be varied for optimalperformance.

The entire color display may be printed using the roll-to-roll processdepicted in FIG. 8 under atmospheric conditions.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention.

What is claimed is:
 1. An illumination structure comprising: a firstconductor surface; a first layer of inorganic first vertical lightemitting diode dies (VLEDs) distributed on the first conductor surfacewith spaces between the first VLEDs, the first VLEDs having a bottomelectrode electrically contacting the first conductor surface; alight-transmissive first dielectric layer laterally between the firstVLEDs; a first transparent conductor layer overlying the first VLEDs andfirst dielectric layer, the first VLEDs having a top electrodeelectrically contacting the first transparent conductor layer; atransparent second dielectric layer overlying the first transparentconductor layer; a second transparent conductor layer overlying thesecond dielectric layer; a second layer of inorganic second VLEDsdistributed on the second transparent conductor layer with spacesbetween the second VLEDs, the second VLEDs having a bottom electrodeelectrically contacting the second transparent conductor layer, thesecond layer of second VLEDs overlying the first layer of first VLEDs; alight-transmissive third dielectric layer laterally between the secondVLEDs; and a third transparent conductor layer overlying the secondVLEDs and third dielectric layer, the second VLEDs having a topelectrode electrically contacting the third transparent conductor layer,wherein the first VLEDs are connected in parallel by the first conductorsurface and the first transparent conductor layer, wherein the secondVLEDs are connected in parallel by the second transparent conductorlayer and the third transparent conductor layer, and wherein the firstVLEDs and the second VLEDs are not vertically aligned to reduce anyblockage of light.
 2. The structure of claim 1 wherein the first VLEDsand the second VLEDs are microscopic VLEDs randomly distributed over thefirst conductor surface and the second transparent conductor layer. 3.The structure of claim 1 wherein the first conductor surface is providedon a flexible substrate.
 4. The structure of claim 1 wherein the thirdtransparent conductor layer is provided on a flexible substrate andlight emitted by the first VLEDs and the second VLEDs exits thesubstrate.
 5. The structure of claim 1 wherein the first conductorsurface is transparent and light emitted by the first VLEDs and thesecond VLEDs exits the first conductor surface.
 6. The structure ofclaim 1 wherein the first VLEDs and the second VLEDs are formed in asingle pixel in a display in an array of pixels.
 7. The structure ofclaim 1 further comprising: a transparent fourth dielectric layeroverlying the third transparent conductor layer; a fourth transparentconductor layer overlying the fourth dielectric layer; a third layer ofinorganic third VLEDs distributed on the fourth transparent conductorlayer with spaces between the third VLEDs, the third VLEDs having abottom electrode electrically contacting the fourth transparentconductor layer, the third layer of third VLEDs overlying the secondlayer of second VLEDs; a light-transmissive fifth dielectric layerlaterally between the third VLEDs; and a fifth transparent conductorlayer overlying the third VLEDs and fifth dielectric layer, the thirdVLEDs having a top electrode electrically contacting the fifthtransparent conductor layer, wherein the third VLEDs are connected inparallel by the fourth transparent conductor layer and the fifthtransparent conductor layer, and wherein the first VLEDs, the secondVLEDs, and the third VLEDs are not vertically aligned.
 8. The structureof claim 7 wherein the first VLEDs, the second VLEDs, and the thirdVLEDs are formed in a single pixel in a display in an array ofsubstantially identical pixels.
 9. The structure of claim 8 wherein thefirst VLEDs emit a first color, the second VLEDs emit a second color,and the third VLEDs emit a third color, wherein the first VLEDs, thesecond VLEDs, and the third VLEDs are controlled to produce an overallcolor output for the pixel.
 10. The structure of claim 8 wherein thenumber of LED dies in each pixel varies due to the LED dies in eachpixel being printed by an LED ink.
 11. The structure of claim 8 whereinthe first VLEDs are printed using a first LED ink, the second VLEDs areprinted using a second LED ink, and the third VLEDs are printed using athird LED ink, wherein the first VLEDs, the second VLEDs, and the thirdVLEDs are randomly distributed laterally in the pixel.
 12. The structureof claim 8 further comprising an addressing circuit for controlling anoverall color of each pixel in the array of pixels.